1. Field of the Invention
The present invention relates to an instant photo film pack that has plural instant photo film units with processing fluid pods and a case body for containing the instant photo film units. More particularly, the present invention relates to an instant photo film pack for preventing rupture of the processing fluid pod by excessive deformation of the case body.
2. Background Arts
An instant camera for use with an instant photo film pack (hereinafter referred to as film pack) is well-known. Japanese Laid-Open Patent Application (JP-A) No. 2000-81688 discloses a film pack having a box-shaped case body and plural instant photo film units (hereinafter referred to as film units) contained in the case. As shown in FIGS. 10 and 11, the case body, comprised of a case body 11 and a rear cover 12, contains a film cover 14, a plurality of film units 15, a first resilient plate 16 and a second resilient plate 17. All elements for comprising the film pack are formed from plastic material having light-tightness. The film unit 15 includes a processing fluid pod 15b, an exposure area 15a and a trap portion 15c, which are provided along an advancing direction of the film unit 15 in this order listed. The processing fluid pod 15b contains processing fluid for developing a latent image, recorded in the exposure area 15a, into a positive image. A surplus of processing fluid is accepted in the trap portion 15b. The processing fluid pod 15b and the trap portion 15c have a thickness larger than that of the exposure area 15a. 
An exposure opening 18 is formed through a front wall 11a of the case body 11, and an ejection slit 19 is formed through an upper wall 11b. The film cover 14 and plural film units 15 are stacked in a stacking direction, and contained in the case body 11 through a rear opening formed in the rear side of the case body 11. The rear opening is covered with the rear cover 12. The exposure opening 18 is covered with the film cover 14, which is ejected out of the case body through the ejection slit 19 when the film pack is loaded in the instant camera. After ejection of the film cover 14, the first and second resilient plates 16 and 17 press the film units 15 toward the front wall 11a, so the exposure area 15a of the uppermost film unit 15 appears through the exposure opening 18. A subject image is exposed in the exposure area 15a through the exposure opening 18, and then the film unit 15 is advanced out of the instant camera through the ejection slit 19. In ejecting the film unit 15, the processing fluid pod 15b is ruptured to spread processing fluid over the whole exposure area 15a, so the positive image is developed in a few minutes.
The ejection slit 19 has a long and narrow opening, and has a size such that the film cover 14 or the film unit 15 is able to pass therethrough. The ejection slit 19 is covered with a resilient light-shielding flap 20 in a light-tight manner. The light-shielding flap 20 is partially stuck to the outer surface of the upper wall 11b, and is partially peeled off to open the ejection slit 19 when the film unit 15 is fed in the advancing direction.
As shown in FIG. 12, the ejection slit 19 has a width larger than the thickness of the film unit 15, so there is a possibility that two or more film units 15 are advanced through the ejection slit 19 simultaneously. In order to prevent this, JP-A No. 2000-89353 discloses a film pack having plural guide projections 22 on an inside of a front wall 11a behind the ejection slit 19, as shown in FIGS. 12 and 13. The guide projections 22 are disposed at certain intervals in the crosswise direction. The guide projection 22 is high in the stacking direction enough so that a top surface thereof is visible through the ejection slit 19. Thereby, the film pack 15 is advanced without being caught by a front slit edge 19a. Moreover, in order to ensure to guide the film unit 15 toward the ejection slit 19, each of the guide projections 22 has a taper inclined toward the front wall 11a. 
For keeping strength, the front wall 11a is required to be formed as thick as possible. In that case, however, since the front case 11 is formed from plastic material by injection molding, the front wall can have molding sink marks. Thus, plural grooves 11c are formed in the front wall 11a for preventing molding sink marks. Between the adjacent grooves 11c, ribs 11d are formed.
However, the exposure opening 18 is larger than the front wall 11a, and the upper wall 11b has a long ejection slit 19. Thus, the front wall 11a does not have strength enough to resist an external force. As shown in FIG. 14, when a strong external force in the stacking direction is applied to the front wall 11a, the front wall 11a is curved inward such that the front slit edge 19a of the ejection slit 19 comes in contact with an opposite rear slit edge 19b. With stronger force, the front wall 11a is deformed such that the front slit edge 19a is located below the rear slit edge 19b, as shown in FIGS. 15 and 16. In that case, the film unit 15 is pressed by the front wall 11a, so the length L of the stacked film units 15 along the stacking direction becomes small. Then, the processing fluid pod 15b can be ruptured to spread processing fluid over the exposure area of unexposed film unit 15, which then becomes useless.
In view of the foregoing, an object of the present invention is to provide an instant photo film pack for preventing rupture of a processing fluid pod in an instant photo film unit, even when an external force is applied to a case body.
To achieve the above objects, an instant photo film pack of the present invention includes an exposure opening formed in a front wall of the film pack, an ejection slit formed in an upper wall of the film pack, at least one engaging projection which is provided on the front slit edge of the ejection slit and extended in the advancing direction, and an engaged portion which is provided in the rear slit edge of the ejection slit. The engaging projection has a shape of a triangular prism, and is provided on the middle of the front slit edge. The engaged portion is provided in the middle of the rear slit edge. When the front wall is deformed to close the ejection slit, the engaging projection presses the engaging portion. The engaged portion is depressed to be engaged with the engaging projection. Thus, it is possible to prevent further deformation of the front wall, which causes rupture of the processing fluid pod in the film unit.
In order to prevent the film unit from being scratched by the engaging projection, the front slit edge has a depression in the center with reference to the crosswise direction of the film unit. A slope is formed in the upper wall, to project the engaged portion toward inside of the case body, and the outer corner of the engaged portion is tapered off. On inside of the front wall behind the ejection slit, plural guide projections for guiding the film unit toward the ejection slit are arranged at certain intervals. A gap is formed between the engaging projection and the guide projection provided behind the engaging projection. When the front wall is deformed excessively, the engaged portion is fit into the gap.
When the engaged portion is depressed by the engaging projection, burrs are formed in the engaged portion. In order to ensure smooth movement of the stack of film units in the stacking direction, a regulating projection for regulating positions of the film units is provided on an inside of the upper wall. Instead of providing the regulating projection, an inner comer of the engaged portion may be tapered off.